Tuneable laser

ABSTRACT

A tuneable laser includes a gain section ( 50 ) bounded by two mirrors ( 51  and  52 ) at least one mirror being a chirp grating. At least one of the mirrors includes a plurality of selectable electrodes ( 59  to  65 ) to enable the grating to be selectively activated to produce a selective reflection at a predetermined wavelength.

This invention relates to tuneable lasers and has particular, but notnecessarily exclusive, reference to tuneable lasers for use intelecommunications systems operating in the C-band, namely within theband of 1530 to 1570 nm.

BACKGROUND OF THE INVENTION

In this specification the term “light” will be used in the sense that itis used in optical systems to mean not just visible light but alsoelectromagnetic radiation having a wavelength between 1000 nanometres(nm) and 3000 nm.

Narrowband lasers are important for a number of applications in opticaltelecommunications and signal processing applications. These includemultiple channel optical telecommunications networks using wavelengthdivision multiplexing (WDM). Such networks can provide advancedfeatures, such as wavelength routing, wavelength conversion, adding anddropping of channels and wavelength manipulation in much the same way asin time slot manipulation in time division multiplexed systems. Many ofthese systems operate in the C-band in the range 1530 to 1570 nm.

Tuneable lasers for use in such optical communications systems,particularly in connection with the WDM telecommunication systems, areknown. A known tuneable system comprises a plurality of individuallywavelength distributed Bragg reflectors (DBR) lasers, which can beindividually selected, or by a wide tuning range tuneable laser that canbe electronically driven to provide the wavelength required. Limitedtuning range tuneable lasers that rely upon thermal effects for tuningare also known.

U.S. Pat. No. 4,896,325 discloses a wavelength tuneable laser havingsampled gratings at the front and rear of its gain region. The gratingsproduce slightly different reflection combs which provide feedback intothe device. The gratings can be current tuned in wavelength with respectto each other. Co-incidence of a maximum from each of the front and reargratings is referred to as a supermode. To switch the device betweensupermodes requires a small incremental electrical current into one ofthe gratings to cause a different pair of maxima to coincide in themanner of a vernier. By applying electrical currents to the two gratingsso that the corresponding maxima track, continuous tuning within asupermode can be achieved. In practice the reflection spectrum of theknown sampled grating structures have a Sinc squared envelope whichlimits the total optical bandwidth over which the laser can reliablyoperate as a single mode device.

In summary, for a given set of drive currents in the front and reargrating sections, there is a simultaneous correspondence in reflectionpeak at only one wavelength, as a consequence of which the device lasesat that wavelength. To change that wavelength a different current isapplied to the front and rear gratings. Thus the front and rear gratingsoperate in a vernier mode, in which the wavelengths of correspondencedetermine a supermode wavelength.

The sampled grating DBR does not have a constant optical cavity lengthas it goes from one supermode to another, which can result in modehopping if great care is not taken to avoid it.

BRIEF SUMMARY OF THE INVENTION

By the present invention there is provided a monolithic tuneable lasercomprising an active section bounded at one end by a first mirror and atthe other end by a second mirror, characterised in that each of themirrors is in the form of a chirp grating, and in that at least one ofthe mirrors has a plurality of selectable electrodes to enable thegrating to be selectively activated to produce a selective reflection ata predetermined wavelength.

The gratings may be located in a material having a refractive indexvariable in response by the passage of a current therethrough and thegrating may be activated at the specific wavelength by the variation ina local region of the refractive index.

The wavelength position of the specific reflection may be altered byvarying the refractive index of at least that region of tie grating andalso the portion of the grating between the region and the gain section

One of the gratings is a front grating and the other a rear grating,with the selectable electrodes being located on either the rear or thefront grating, or both. The pitch characteristics of the front and rearchirp gratings, being the grating pitch against distance along thegrating, may be substantially identical and the chirp gratings may belinear. In a preferred embodiment having minimum or no mode hoppingduring timing and when used at different wavelengths, the two chirpprofiles have different chirp characteristics so that the optical cavitylength is constant or substantially constant at different wavelengths.

The reflectivity of the rear mirror may be greater then the reflectivityof the front mirror.

The front and rear mirrors may be formed by electron beam writing of thegrating patterns and the mark space ratio of the rear mirror may besubstantially unity and the mark space ratio of the front mirror may besubstantially different to unity.

The reflectivity of the rear mirror is typically 50% and thereflectivity of the front mirror is typically 30%.

The pitch spacings of the rear mirror chirp may be at its lowestadjacent the gain section and the pitch spacings of the front mirror maybe at its highest adjacent the gain section.

There may be provided a phase change section between the gain sectionand the rear grating.

One or more of the tuning electrodes within the optical cavity may beutilised as a phase control means to reduce or eliminate mode hopping.

Light power may be coupled out from both ends of the laser, and bothgratings may be substantially identical.

The composition of the mirror sections typically is formed of GroupIII-V semiconductor layers of different refractive index.

The present invention also provides a method of operating a laser inwhich the rear grating has a plurality of selectable electrodes and inwhich the selection of a wavelength occurs by passing current throughone of the electrodes to reduce the refractive index of the portion ofthe chirp grating affected by the electrode in which those electrodescapable of reducing the refractive index of the portion of the chirpeffective at a shorter wavelength are also actuated to prevent theformation of a distorted reflection peak.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a and 1 b show the concept of a chirp grating,

FIG. 2 shows a wave front passing along a chirp grating,

FIG. 3 is a pair of box diagrams of intensity against wavelength,

FIG. 4 is laser assembly with two chirp gratings,

FIG. 5 shows the chirp characteristics of the laser of FIG. 4

FIG. 6 a shows the laser of FIG. 4 with castellated gratings

FIG. 6 b shows the reflectivity of the gratings of FIG. 6 a

FIGS. 7 and 8 illustrate the effect of light passing in both directionsalong strong and weak gratings,

FIG. 9 is a schematic view of a laser in accordance with one aspect ofthe invention,

FIG. 10 shows the laser of FIG. 9 with its chirp characteristics andchirp reflectivities,

FIG. 11 shows the laser of FIG. 10 with an enlarged view of a portion ofthe rear chirp characteristics,

FIGS. 12 and 13 are schematic views of a tuning diagram for a rearmirror,

FIG. 14 is a graph of reflectivity against distance,

FIG. 15 is a schematic view of the profiled tuning arrangement of therear mirror, and

FIGS. 16 and 17 are schematic sectional views of two further embodimentsof the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

The wavelengths of interest referred to above, for example the C-bandwavelengths of 1530 to 1570 nm are the wavelengths of light in freespace. When such light passes through a medium, of refractive indexn_(eff), the wavelength of the light changes. The actual wavelength ofthe light within that medium which will be referred to herein as λ′, isthe wavelength λ divided by the value for the refractive index n_(eff).In other wordsλ′=λ/n _(eff)   (1)where n_(eff) is the effective refractive index of the medium as seen bythe propagating light.

It so happens that the glass (silica) fibres, which are commonly used intelecommunicatons systems, have low loss regions at about 1100 nm 1300nm and 1500 nm. These regions are about 100 nm wide and consequentlymuch work is done on producing lasers that produce light in the low lossregions The same is true for the tuneable laser of the presentinvention. The specific examples of the invention are designed to workin the C-Band, but the invention could be used for other wavelengths ifrequired and if new types of fibre optical

les become available.

Referring to FIG. 1 a, this shows the nature of a chirp grating, as theterm is used in the specification and as it is to be understood herein.The chirp grating is a form of Bragg grating which has a substantiallycontinuous variation in the wavelength at which it reflects light alongits length. It is thus distinguished from a normal DBR, which reflectsat a single wavelength, and also from a sampled grating DBR whichreflects at a plurality of discrete wavelengths.

A chirp grating is formed at the interface between two materials ofdifferent refractive index and can be represented graphically as asinusoidal shaped waveform, or as a castellated form. The physical shapeof the grating is dependent upon the etching technique employed and mayresult in a castellated form, particularly when a dry etching process isused to produce the grating, e.g. reactive ion etching.

As shown in FIG. 1 a, the grating is formed as an interface 1 betweenthe upper layer of material 2 of a low refractive index and a lowerlayer 3 of a higher refractive index. The pitch Λ of the grating isgradually increased along the length of the grating from Λ_(x) at theshort pitch end of the chirp grating to Λ_(L) at the long pitch end ofthe grating. In FIG. 1 a the increase in pitch is deliberatelyexaggerated to demonstrate what is happening. In practise, for typicaltelecommunications bands, the increase in pitch length over the whole ofthe grating will be small, namely about 2.5%. This means that at theshort end the grating reflects light of a wavelength corresponding to1530 nm and at the long end the grating reflects light at a wavelengthcorresponding to 1570 nm. Thus there is a 40 nm variation in thereflection wavelength over the length of the grating, which is about2.5% of the average wavelength of 1550 nm.

The refractive index n of the material used in the production of thechirp grating through which the majority of the light passes isquaternary material (InGaAsP) and the refractive index n of the materialvaries, as stated above, with the wavelength of light passing throughthe material. Typically n at 1570 nm is 3.33, at 1550 nm n is 3.38 andat 1530 nm n is 3.43. Thus n decreases by about 3% from 1530 nm to 1570nm.

In FIG. 1 b, there is a graph showing how the pitch of the gratingvaries along its length, with the pitch, Λ, in the vertical axis and thelength of the grating x on the horizontal axis.

It will be appreciated that the pitch values, Λ, along the length of thegrating can be plotted directly against the length and a line 4 isgenerated. The line 4 can be straight or can be curved depending on howthe pitch is varied along the length of the grating. If the increase ingrating pitch is at a constant rate, the line is straight and thegrating is called a linear chirp grating. If the increase in gratingpitch along the grating is uniform, in other words in the direction ofincreasing Λ, each Λ is a certain small constant amount greater inlength than the one before it, then in this case line 4 will not belinear but will curve downwards as the line increasingly goes to theright as shown in the drawing at 4 a.

Referring to FIG. 2, this demonstrates the effect of light passing alonga chirp grating. Again the grating is shown as a sinusoidal interface 5between an upper layer 6 of a lower refractive index and a lower layer 7of higher refractive index. The waveguide of the assembly of highrefractive index is shown at 8, separated from the lower layer 7 of thechirp grating by an intermediate layer 10 of low refractive index.Underneath the waveguide 8 is a low refractive index substrate 11. Thetwo layers 10 and 11 serve to contain the light passing through thewaveguide 8. Superimposed on the layer structure is a graphicalrepresentation of the wave front of the light passing through from leftto right as at 12 in the direction of the arrows 12 a. Trace 13 is anindication of the intensity of the light in the layers of the assemblywith the higher intensity levels to the right of the figure. It can beseen that most of the light passes through the waveguide 8 of highrefractive index.

As shown in FIG. 2, the light passes not only through the waveguide 8but also through the layer 7 forming the lower layer of the chirpgrating. If the light should happen to have a wavelength λ′ which istwice the grating pitch Λ then that light will be reflected back i.e. ifλ′=2Λ then that wavelength of light will be reflected. Thus the chirpgrating as a whole will reflect light in the range λ′_(s)=2Λ_(s) toλ′_(L)=2Λ_(L) where Λ_(s) , is the shortest grating pitch and Λ_(L) isthe longest grating pitch. Light of wavelengths outside of this rangewill not be reflected back along the waveguide.

This can be represented diagrammatically as in FIG. 3, which are boxdiagrams of intensity of light, I, in the vertical axis and wavelength,λ′, in the horizontal axis. If a box of light consisting of a pluralityof wavelengths is admitted into the grating as shown at 14 on the lefthand side of the drawing, the envelope is complete and represents all ofthe wavelengths between λ′₁ and λ′₂, which are widely, separatedwavelengths. However as the chirp grating reflects certain of thewavelengths, for example between λ′_(s) and λ′_(L), the emerging box ofwavelengths 15 has a gap 16 which corresponds to those wavelengthsbetween λ′_(s) and λ′_(L) reflected by the chirp grating.

As illustrated in FIG. 4, for the purposes of explanation, two chirpgratings shown generally by 17 and 18 can be created on either side of again section 19. Initially, for the ease of understanding of theinvention, the gain section will be ignored. In FIG. 5, the two chirpgratings 17 and 18 are shown superimposed on the pitch vs distance plots20 and 21, which form the lower portion of FIG. 5. These pitch vsdistance plots show pitch Λ in the vertical axis and distance x alongthe grating in the horizontal axis. The grating 17 will be termed thefront grating and the grating 18 will be termed the rear grating. Theshape of the graph of pitch spacing from Λ_(S) to Λ_(L) vs. distance asrepresented by trace 22 for grating 17 is the same as that for grating18 as shown by trace 23.

As mentioned above, the interface between the materials of differentrefractive index which goes to make up the chirp grating, can beproduced in the form of castellations, and are illustrated in FIG. 6 aat 24 and 25. The preferred method of producing the castellations is toproduce layer 7 and to etch grooves into the layer and then to growlayer 6 on top of the layer 7 containing the grooves. The width of thegrooves is then conventionally referred to as a space, S, and thedistance between the grooves is referred to as a mark, M. The width ofthe mark M and the spacing S can be varied, so as to alter themark/space ratio (M/S ratio). The most reflective form of the grating iswhen M/S=1, and as the ratio of M/S varies away from 1 the gratingbecomes less reflective. The rear grating has a mark/space ratio of 1whereas the front grating has a mark/space ratio greater or less than 1.

The effect of this difference is shown schematically in the lowestportion of FIG. 6 b, which is a graph of reflection η against distancex, and it can be seen that the front chirp has a reflection coefficientof about 30% and the rear grating has a reflection coefficient of about50%. This is shown by the traces 26 for the front grating and 27 for therear grating.

The front and rear chirp gratings can be either of the same chirpprofile, or have different profiles, for reasons that are set out in alater part of the specification. If the chirp gratings have the sameprofile then a phase section functionality is required. In thecircumstance where the front and rear chirps have the same chirp profilethen this means that the slopes 22 and 23 are identical, as shown inFIG. 5. The distance 28 between two corresponding positions 33, 34 atthe short ends of the gratings, as shown in FIG. 4, will be the same asthe distance 29 between two corresponding positions 35, 36 at the longends of the gratings. The significance of this will be seen below.

The effect of placing a gain medium in the region 10 is that light canbe generated in that region and this generated light will initially passoutwards in both directions from the centre.

Light which is of a longer wavelength than that which can be reflectedby the grating at any point does not in effect “see” the grating and isunaffected by it. Thus light of a wavelength longer than λ′_(L) will besubstantially unaffected by the grating but be partially absorbed by thewaveguide, and light of shorter wavelengths will be variously affected.With reference to FIG. 4, light having wavelengths between λ′_(L) andλ′_(S) passing to the left of the gain section in the direction of thefront grating will be reflected back as at 30 to 31.

Light of the low wavelengths λ′_(L) will be reflected immediately by thegratings of pitch fr

Λ_(L), but the light of the shorter wavelengths λ′_(S) is partiallyattenuated by having to pass through the longer pitch gratings on itsway to reach the front grating of pitch λ_(S) and be reflected. Thatlight then has to pass back through the longer pitch gratings and isagain partially attenuated. Thus from the front grating as a whole, thereflected light will have the longer wavelengths predominating, as theshorter wavelengths are more attenuated by the grating.

The light passing to the right into the rear grating will also bereflected back but as the shorter wavelengths are reflected first, anddo not have to pass through the longer wavelength sections of thegrating, they are not attenuated to the same extent as light of the samewavelengths passing through the front grating. Again the light of longerwavelengths, which is reflected by the rear gratings of pitch Λ_(L),does not “see” the shorter pitch gratings. Thus, the light at allwavelengths is reflected more uniformly by the rear grating than by thefront grating.

The asymmetric effect of attenuation on the reflected light leads topreferred directions for the passage of light through the front and rearchirps. This is illustrated in FIGS. 7 and 8.

Ignoring waveguide losses FIG. 7 illustrates what happens with a higherstrength reflecting grating. In the upper two sections of the drawinglabelled Case A the reflection of a box of wavelengths passing along achirp grating 40, from shorter to longer pitch in the direction 41 isillustrated at 42 in the box diagram of light intensity I vs. distancex. The reflection is substantially the same at the shorter wavelengthsλ′_(S) as at the longer wavelengths λ′_(L). However if the light ispassed through the same grating 40 in the other direction, as in thedirection of arrow 43 as shown

belled Case B, then the short wavelengths λ′_(S) are reflected to a farlesser e

nt than the longer wavelengths λ′_(L). This is as shown in the lower boxdiagram at 44. It can be seen that the shorter wavelengths are stronglyattenuated compared to the longer wavelengths.

By comparison the reflection effects for a lower strength reflectinggrating 45 are shown in FIG. 8. Again the light passing in the direction46 from the shorter pitch to the longer and labelled Case A is reflectedsubstantially uniformly across all the wavelengths as shown at 47 in thebox diagram of light intensity I vs. distance x. However the shorterwavelengths of light passing in the opposite direction 48, and labelledCase B, are attenuated to a lesser extent compared to the grating ofstronger reflectivity, as shown in the box diagram by line 49.

As a consequence in a preferred embodiment the rear strong mirror hasΛ_(S) adjacent the gain section and the front mirror has Λ_(L) adjacentthe gain section.

FIG. 9 illustrates in cross section the upper portion a laser assemblyin accordance with one embodiment of the invention. The laser comprisesa four-part assembly of a central gain section 50 with a front mirror inthe form of a chirp grating 51 and a rear mirror in the form of a chirpgrating 52. There is also a phase change section 53, the function ofwhich will be described below.

FIG. 10 is a plot of grating pitch Λ against distance x, for anembodiment where the front and rear gratings have identical chirpprofiles. This is shown by the traces 54, 55. It will be seen that trace55 has a region 56, shown dotted below the main portion of the trace,for reasons which will be explained below.

On the outer surface of the laser there are a series of electrodes 57 to65. The electrode 57 can be used to inject current into the gain sectionto make it create light. The electrode 58 can be used to control thephase section as described below and the electrodes 59 to 65 are able toinject current into different regions of the rear grating 52.

If just sufficient current is injected into the gain section to make itgenerate light, then if the chirp sections are capable of reflectinglight in the range of 1530 to 1570 nm the wavelengths of light withinthat range will be internally reflected. Light outside of the reflectingwavelengths will be absorbed or will be emitted from the ends of thelaser. The laser will not lase because the intensity of the light at allof the frequencies in the range 1530 to 1570 nm is below the lasingthreshold.

To get the laser to lase, it is necessary to have both a populationinversion of charge carriers within the gain material and to get atleast one, and preferably only one, wavelength to be above the lasingthreshold. This is achieved by injecting sufficient current into thegain section 50 through electrode 57 to create the population inversionand by making a portion of the rear grating reflect light of a specificwavelength preferentially, so that the rear grating selectively reflectslight of that particular wavelength. The front grating will reflect backthe light of that wavelength, so that that wavelength will become thepreferred or enhanced wavelength and the laser will commence to lase atthat wavelength.

The selection of the particular wavelength is effected by passing acurrent through an electrode such as electrode 62 above the portion ofthe chirp grating which corresponds to the region 56 in the trace 55.The effect of the passage of current is to increase the current densityin that region of the grating, which lowers the refractive index of thegrating layer 7 just below the electrode 62. The lowering of therefractive index has the effect of making the grating reflect at a lowerwavelength, which is the same effect as would be obtained by shorteningthe grating pitches in that region.

This means that the effective grating pitches of the dotted portion 56as is shown in the central portion of FIG. 10, now line up with theadjacent region 66, forming a chirped Fabry-Perot étalon, which ideallyreinforces the reflection in the adjacent region 66.

Referring to the lowest portion of FIG. 10, which is a graph ofreflectivity η vs. distance x, it can be seen that although thereflectivity of the front grating 67 remains flat there is a trough 68in the reflectivity of the rear grating which corresponds to the region70 that now reflects at a lower wavelength. However there is now anenhancement of the reflectivity of the region 66 due to the resonantchirped Fabry-Perot étalon structure as shown in FIG. 11. It can be seenthat the lines 55 and 56 form a small version of the large parallelarrangement of the chirp diagrams. Thus there is produced a reinforcedpeak 69 in the reflectivity as seen in FIG. 10.

Light at the wavelength that corresponds to the position of peak 69 isthus selectively reflected and the laser commences to lase at thatwavelength.

It will be appreciated that without any further adjustments the lasercould only be tuned to as many different wavelengths as there areelectrodes 59 to 65.

However, the device can be made continuously tuneable if the materialsfrom which the chirp gratings are constructed have a sufficientlyvariable refractive index.

FIG. 12 illustrates how this can be put into effect. In FIG. 12 there isshown the rear mirror chirp grating under three different conditions.

In the drawing there are shown ten electrode positions 100 to 109, whichcorrespond to the electrode positions 59 to 65 in FIG. 10. In otherwords, instead of there being seven electrodes over the rear grating,there are ten electrode positions in this schematic. The line 110corresponds to the line 55 of the rear grating as shown in FIG 10. Thevertical dotted lines show the alignments of the electrodes and theportions of the chirp diagram.

In the upper portion of the FIG. 12 there is no current flowing throughany of the electrodes 100 to 109. The line 110 is continuous and thefront and rear chirp gratings are in the same state with no portionbeing preferred.

In the central portion of FIG. 12 a current is passed through electrode106. The current is half that required to cause the maximum reduction inthe refractive index, n, of the material of the chirp grating below theelectrode 106, which is equivalent to material 7 in FIG. 10. The resultof this is to displace downwards the portion 111 of the line 110. Thisresults in a selection of a particular wavelength at which the laser canlase in exactly the same manner as described above with reference toFIG. 10.

To further tune the laser, so as to reduce the wavelength at which l

g is preferred, current is passed through all of electrodes 100 to 105and at the same time the current passing through electrode 106 isincreased. This causes a lowering of the portion 112 of the chirp linebelow its original position, shown dotted. The portion 111 a a of theline 110 also is lowered at the same time, thus moving the point ofselection to a lower wavelength. In best practise no additional currentneed be passed through electrodes 107 to 109, as they play no part inthe reflecting process. However, since they play no part in theselection process, it is possible for the electrodes 107 to 109 to belowered in amounts similar to electrodes 100 to 105 without interferingwith the wavelength selectivity. When the current passing through theelectrode 106 is the maximum which can be applied to reduce n, and thusthe maximum amount of fine tuning has occurred, the electrodes 100 to105 will be passing a current which corresponds to half of the totalreduction of n in the material in layer 7 below electrodes 100 to 105.

To further tune the laser, the current is removed from electrode 106 andis applied to the next adjacent electrode (or any other selectedelectrode) and the sequence of actions is repeated. By this means thelaser can be tuned over the entire 1530 nm to 1570 nm waveband.

The selectivity of the chirp at a particular wavelength can be enhancedas shown schematically in FIG. 13. This Figure is closely similar toFIG. 12 but shows what happens when two adjacent sections of the chirpgrating are moved together.

In the upper portion of FIG. 13 the rear chirp grating is shown in thesame position as in FIG. 12. This is also the case for the centralportion of FIG. 13, where current applied to electrode 106 has caused alowering of the line 11 to the position half way down to its maximumextent. If the current is passed through electrode 105 this causes theline 113 to be lowered and the current passing through electrode 106 isincreased at the same rate so that lines 111 a and 113 move down insynchronism. This means that the grating selectivity is increased by theenhanced reflectivity.

When the applied current to electrode 105 is half of that applied toelectrode 106 and the line 111 a is depressed to its maximum extent thelines 111 a and 113 will also coincide with portion 114 of line 110 togive a three-region coincidence.

When all of the sections have lined up as shown in the lower section ofFIG. 13 there is created a clean reflection peak as shown at 115 in theleft portion of FIG. 14, which is a graph similar to the lower portionof FIG. 10. However, during the tuning process as 113 and 111 a “slide”over 114, a partial second étalon is created causing a perturbation inthe primary curve, producing a distorted reflection as shown at 116 inthe right hand portion of FIG. 14.

To prevent this two, three or more electrodes on the lower wavelengthside of the wavelength selecting electrodes are lowered as shown in FIG.15. In this case each of the 104, 103 and 102 electrodes have justsufficient current injection to avoid degradation of the reflectionpeak. Typically the current in lower wavelength adjacent electrodes willbe in excess of half that of the higher wavelength adjacent electrode.For simplicity these profiled currents are referred to as half currents.If the reflection peak is considered to be centrally located at themid-point of 111 a and 113, then the centre of 104 a is below this peakand in effect out of the way. By lowering portion 103 a by half of thelowering of 104 a and so on, there is no substantial reinforcementbetween sections 104 a and 103 a. This means that only sections 111 aand 113 are in reinforcement and the unwanted perturbations as seen inFIG. 14 on the right, 116, do not occur.

It will be appreciated that the more electrodes that can be installedover the chirp, the greater the number of regions that can be broughtinto coincidence and the smaller each reduction in refractive indexneeded at any point to tune the laser. For example, for a range of 40 nmtotal tuning, if twelve different electrode positions are used, theneach is only required to tune through a range of 4 nm, whilst ensuringthe desired band coverage is obtained.

By this process, therefore, the laser can be tuned over the whole of thedesired wavelength range, depending on the bandwidth of the originalgratings.

It will be appreciated that during the coarse tuning the peaks atdifferent wavelengths will always lase by reflecting with the frontmirror at exactly the same wavelength, for the reasons set out above.For identical front and rear chirps this would mean that the physicallength which the light has to travel would be the same irrespective ofwhether the laser was lasing at short or long wavelengths. In otherwords the physical length 28 would be identical to the physical length29 as shown in FIG. 4.

However, as the refractive index of the material through which the lightis traveling varies with wavelength then this would mean that the lightlasing at the shorter wavelengths has a different optical cavity lengthto the light at the longer wavelengths.

What is meant in this specification by a constant optical cavity lengthis a length in which a constant number of unit standing wave periods canbe supported. The number of standing waves, which can be supportedwithin the laser assembly is given byN=2.n.L/λ  (2)

Where N is the number of standing wave periods, which can be supported,L is the physical cavity length and n is the cavity refractive index.

This means that for identical front and rear chirps the number ofstanding wave periods varies with the wavelength at which the laser isoperating and there will mode hopping. The function of the phase changesection 53 controllable by electrode 58 is to give a constant opticalcavity length for the embodiment of the invention shown in FIG. 9. Thephase change section can also be used to compensate for ageing of thelaser. It can also be used to correct small changes occurring as aresult of thermal effects.

A number of ways of substantially eliminating or ameliorating theoptical cavity length change have been developed. In addition a numberof alternative methods of operating the laser to effect optimumperformance have also been developed. A way to substantially eliminatethe change in the optical cavity length, as the laser is tuned todifferent wavelengths, is to design and construct the chirp gratings sothat as they are written, the chirp characteristics of the front andrear chirps are slightly different so that the number of standing wavesas set out in the above equation (2) is the same irrespective of thewavelength selected. Tuning is thus carried out in a system in which thenumber of standing waves, N, is constant or as near constant as can bemanufactured so that the laser is not prone to mode hopping and littlephase control is required. This constitutes optimised chirp gratings forfront and rear mirrors.

The shorter the laser the lower the losses there are in the laser, thelarger are the longitudinal mode spacings and the easier it is to selectthe different wavelengths. Thus shortness is a distinct advantage and ithas been found that the small amount of phase change needed for thelaser of the invention is such that it may be carried out by using therear mirror as a combined reflector and phase change section. The lesscurrent there is injected into the mirror section the lower thewaveguide losses introduced. It is for this reason that the preferredembodiment incorporates current profiling across adjacent gratingelectrodes. Two embodiments of this aspect of the invention areillustrated in FIGS. 16 and 17.

Referring firstly to FIG. 16 this shows a chirped laser assembly inaccordance with the invention in which there is provided a central gainsection 140 having on one side a rear chirped mirror 141 and on theother side a front chirped mirror 142. The gain section 140 and the rearmirror section 14 are provided With electrodes 143 to 150 which functionin the same manner as described above for electrodes 57 and 59 to 65 inFIG. 9. In the laser shown in FIG. 16, there is an electrode 151 overthe front mirror which can be used to inject current into the frontmirror. This introduces a change of refractive index over the whole ofthe front mirror so as to cause a variation in the effective opticalcavity length. A virtue of this laser design is that it is a mode hopfree design brought about without the need to introduce a separate phasecontrol section. The integration of the mirror with a phase controlfunctionality gives advantages of shorter laser cavity length and areduction in the current induced waveguide losses.

Although the use of a single electrode on the front mirror requires asimple form of current switching there is however, an increased risk ofcurrent induced waveguide losses. In the embodiment shown in FIG. 17,the use of segmented electrodes facilitates minimal current inducedwaveguide losses.

As shown in FIG. 17 there is again a gain section 160 a rear mirror 161and a front mirror 162. The gain section has an electrode 163 and therear mirror has electrodes 164 to 170. In the case of this embodimenthowever, the front mirror 162 has a series of electrodes 171 to 177,which can be individually selected. Any available tuning electrodeswithin the lasing optical cavity can be utilised as a phase controlmeans to reduce or eliminate mode hoping. The segmented electrode frontmirror arrangement is preferably used in combination with the optimisedchirp gratings for front and rear mirrors.

Within the scope of the invention there can be embodiments in which thefront chirp grating has a single electrode whilst the rear chirp gratinghas multiple electrodes; embodiments in which the front chirp gratinghas multiple electrodes and the rear chirp grating has a singleelectrode; and further embodiments in which both the front and rearchirp gratings have multiple electrodes. In each of these embodimentsthe strengths of the gratings can be strong or weak in dependence of thelaser end from which it is desired to couple out light power.

In the symmetrical design case, wherein both the front and rear chirpgratings are each equipped with multiple gratings, it is possible forthe gratings to be of equal length such that light power may be coupledout from both ends at substantially the same power level. This might bean advantage where the laser is required to feed two different paths,and avoids the need to provide an external power splitting means. It isfrequently the case that a laser has light coupled out at both ends but,normally one of these out couplings is low power for monitoring purposesonly.

In practice waveguide losses may be balanced by controlling the markspace ratio along the chirp gratings, or by adjusting the chirp profilesof the front and rear mirror in a corresponding manner.

1. A monolithic tuneable laser, comprising a gain section bounded at oneend by a first mirror and at the other end by a second mirror, whereineach of the mirrors is in the form of a chirp grating, and wherein atleast one of the mirrors has a plurality of selectable electrodes toenable the chirp grating to be selectively activated to produce aselective reflection at a predetermined wavelength.
 2. The laseraccording to claim 1, wherein the chirp gratings are located in amaterial having a refractive index variable in response to the passageof a current there through, and wherein the chirp grating is activatedat the predetermined wavelength by the variation in a local region ofthe refractive index.
 3. The laser according to claim 1, wherein awavelength position of the reflection is altered by varying a refractiveindex of at least that region of the grating and a portion of thegrating between said region and the gain section.
 4. The laser accordingto claim 1, wherein one of the chirp gratings has a plurality ofselectable electrodes and the other chirp grating has a singleelectrode.
 5. The laser according to claim 1, wherein one of the chirpgratings is a front grating and the other chirp grating is a reargrating, and wherein the selectable electrodes are located on the reargrating.
 6. The laser according to claim 5, wherein each of the frontgrating and the rear grating have a pitch characteristic and wherein thepitch characteristics of the front and rear chirp gratings aresubstantially identical.
 7. The laser according to claim 5, wherein eachof the front grating and the rear grating have a pitch characteristic,and wherein the pitch characteristics of the front and rear chirpgratings are such that the optical cavity length of the laser issubstantially constant at different wavelengths.
 8. The laser accordingto claim 1, wherein the chirp gratings are linear.
 9. The laseraccording to claim 1, wherein a reflectivity of one of the first andsecond mirrors is greater than a reflectivity of the other mirror. 10.The laser according to claim 1, wherein the first and second mirrors areformed by electron beam writing of the grating patterns.
 11. The laseraccording to claim 10, wherein a mark space ratio of one of the firstand second mirrors is substantially unity and a mark space ratio of theother mirror is different than unity.
 12. The laser according to claim9, wherein the reflectivity of one of the first and second mirrors is ofthe order of 50% and the reflectivity of the other mirror is of theorder of 30%.
 13. The laser according to claim 1, wherein pitch spacingsof one of the first and second mirrors is lowest adjacent the gainsection and the pitch spacings of the other mirror is highest adjacentthe gain section.
 14. The laser according to claim 1, further comprisinga phase change section disposed between the gain section and one of thefirst and second mirrors.
 15. The laser according to claim 1, wherein acomposition of the first and second mirrors is formed of Group III-Vsemiconductor layers of different refractive index.
 16. The laseraccording to claim 1, wherein both of the first and second mirrors havea plurality of electrodes, so as permit both mirrors to be selectivelyactivated to produce a selective reflection at the predeterminedwavelength.
 17. The tuneable laser accordingly claim 16 wherein at leastone of the plurality of electrodes comprises a tuning electrode disposedwithin an optical cavity, wherein the tuning electrode is employed as aphase controller to reduce or eliminate mode hopping.
 18. The tuneablelaser according to claim 1, wherein light power is coupled out from bothends of the laser.
 19. The tuneable laser according to claim 18, whereinboth chirp gratings are substantially identical.
 20. A method ofoperating a laser as claimed in claim 1, comprising the steps ofselecting a wavelength by passing current through one of the pluralityof electrodes to reduce a refractive index of a portion of the chirpgrating affected by the electrode, and actuating one or more electrodesof said plurality of electrodes capable of reducing the refractive indexof the portion of the chirp grating effective at a shorter wavelength toprevent the formation of a distorted reflection peak.
 21. (Canceled)